Engagement of the multicomponent antigen receptor in T cells (TCR) results in rapid activation of a protein tyrosine kinase pathway. A major TCR- associated protein tyrosine kinase is ZAP-70, a protein that binds to the activated TCR. Under conditions of TCR activation, the ZAP-70 bound to the TCR is itself tyrosine phosphorylated and activated. Use of a T cell line that lacks this enzyme has provided more evidence for the central role of this kinase. In the absence of this enzyme, TCR-mediated signaling is grossly inhibited. Reconstitution of this cell line with normal, but not defective forms of this enzyme resulted in restoration of signaling and T cell function. Additional analysis of ZAP-70 has been performed in studies in which a chimeric protein comprised of ZAP-70 and the green fluorescent protein has been expressed. With this molecule we have been able to study the dynamic movement of ZAP-70 translocation from cytoplasm to membrane, and have been able to identify that a fraction of ZAP-70 is found in the nucleus. Additional studies focus on a substrate of tyrosine kinases in T cells, p120cbl (Cbl), a proto-oncogene, which can be found in complex with a number of critical signaling molecules. Engagement of the Fc receptor for IgE on mast cells also results in Cbl tyrosine phosphorylation. In these cells we have demonstrated that the Syk protein tyrosine kinase is primarily responsible for this phosphorylation. Moreover, Syk and Cbl can be found in a complex. The effect of Cbl binding is to inhibit Syk interaction with the Fc receptor. Consequently Syk autophosphorylation and kinase activity is inhibited thus resulting in an inhibition of intracellular tyrosine phosphorylation and serotonin release from these cells. These experiments demonstrate that the Cbl protein may serve as more than just a simple scaffold protein; instead, interaction with Cbl and other similar adaptor proteins may directly regulate enzyme localization and function.